Leaching of heavy metal bearing wastes and direct discharges of heavy metal bearing waste waters has been of concern to environmental regulators, waste producers and various health officials since the 1970's. This concern resulted in the promulgation of the Resource Conservation and Recovery Act (RCRA) in 1979. Under RCRA, solid wastes may be considered hazardous if the waste leaches excessive heavy metals under the Toxicity Characteristic Leaching Procedure (TCLP) as set forth in the Federal Register, Vol. 55, No. 61 (Mar. 29, 1990) and which corresponds in pertinent part to the procedure set forth in the Federal Register, Vol. 55, No. 126, pp.26985-26998 (Jun. 29, 1990). In addition, there exist various states such as California, Michigan and Vermont which require additional leaching tests on solid waste in order to classify the waste and direct the more heavy metal leaching wastes to hazardous waste landfills.
In order to avoid having solid wastes be required to be handled at more expensive hazardous waste landfills, various researchers and solid waste businesses have investigated and developed methods to control the leaching of heavy metals, such as lead from the solid waste. The art has looked at the control of leaching by ex-situ methods involving portland cement, silicates, sulfates, phosphates and combinations thereof. See U.S. Pat. Nos. 4,629,509 (calcium sulfide); 4,726,710 (sodium sulfur oxide salt); the teachings of which are incorporated herein by reference in their entirety.